Though numerous applications utilize coiled heat exchangers, the gas turbine recuperator is among the most demanding. In any application, and especially when used as a gas turbine recuperator, the heat exchanger should be compact, efficient, reliable, and relatively inexpensive to manufacture. By designing the primary heat transfer surface with small hydraulic diameters and counterflow circulation of heat transfer fluids, a relatively compact and efficient heat exchanger can be obtained. Furthermore, providing the heat exchanger with relatively large cross-sectional flow areas reduces load losses. Achievement of large cross-sectional flow areas in coiled heat exchangers requires circulating heat transfer fluids in the axial, as opposed to tangential, direction. Additionally, production costs can be lowered by minimizing the number of elements used to make the heat exchanger and by forming and coiling the heat exchanger in a continuous process. Another design consideration, especially when used as a gas turbine recuperator, includes resistance to thermal shock. Heavy thermal loads often result from the transient operation of turbines. Therefore, to ensure reliable performance and operation, the heat exchanger should have high resistance to
Various known heat exchangers are made from coiling a pair of sheets between which heat transfer fluids circulate in a counterflow manner in directions substantially parallel to the longitudinal axis of the coil. For example, U.S. Pat. No. 5,797,449 pertains to an annular heat exchanger formed by a pair of sheets welded together and coiled, with openings cut through the sheets through which heat transfer fluid passes.
German patents DE 1121090 and DE 3234878 describe spiral heat exchangers having axially circulated fluid flows, in which the fluids enter and exit through alternating angular sectors. In DE 1121090, sectors for circulating the heat transfer fluids are formed by cutting evenly-spaced openings in borders that close the edges of a pair of sheets coiled to form the heat exchanger. After the borders are cut, the two sheets are coiled to form the heat exchanger. DE 1121090 additionally discloses the fabrication of the spiral heat exchangers with external headers.
In DE 3234878, the sectors are formed by glueing blocking segments on the two faces of the coiled heat exchanger.
Finally, in French patent document FR-A-231 9868, borders are closed by the direct welding of adjacent sheets.
A particular difficulty in heat exchangers having a coiled configuration includes the distribution of the single incoming flow into the myriad of small heat transfer passages and the collection of the same into a single outgoing flow after the heat transfer has taken place. Preferably, this distributing and collecting should not result in excessive head losses, nor should it cause mechanical stresses due to large thermal gradients. Another difficulty arises from blockages to the heat transfer fluids that exist on the core face as the result of the particular construction used for the heat exchanger.
For instance, in one known example, the sheets are constructed and cut such that one sheet has openings only for one fluid and the other has openings only for the other fluid. This leads to a relatively high amount of fluid being blocked at the core faces, thus reducing gas flow passage and overall efficiency of the heat exchanger.
Stacked plate heat exchangers often include openings cut in the plates to distribute and collect the heat transfer fluids. The edges of these openings generally are either brazed or welded together during assembly of the heat exchanger (for example in U.S. Pat. No. 4,073,340) or are fitted with a gasket (for example in Alfa-Laval plate heat exchangers). Other stacked plate heat exchangers do not include such openings (see SAE 851254. "Development, Fabrication, and Application of a Primary Surface Gas Turbine Recuperator", E. L. Parsons), but the sides of the plates must be provided with sealing bars.
Also important in constructing a coiled heat exchanger is the connection of the external headers with the core. The header-to-core connection must be sealed to prevent leakage of heat transfer fluids being passed to the heat exchanger core. Furthermore, headers should have the strength to resist forces tending to pull them away from the core due to the relatively high pressures experienced as fluids are collected and distributed and to inertial forces resulting from supporting the core weight. Additionally, temperature gradients occurring between the core and the header can result due to sudden transient temperatures in one of the heat transfer fluids combined with the relative thermal inertia of the core and the headers. Such gradients may cause thermal expansion forces on headers. Therefore, construction of the heat exchanger needs to account for these effects as well.